"See you later!"
"Gxis" should be acceptable too, no? Lost me a point, but -x for the caret is pretty widely known and used in Esperantujo, or so I thought :O
If you mean just ‘ĝis’ then it says ‘ĝis on its own means “bye”’. It should probably give the same hint in both cases though.
(IMO ‘bye’ and ‘see you later’ are synonyms anyway but whatever.)
The added 'x' is a typing shorthand or code. So you don't have to have a special font or character keyboard. However, since Duolingo has the character inputs (buttons below the input box), they likely don't use this. And, imo, it's better to learn the actual character. A bit like the "imo" I just used, it's fine for casual internet use, but otherwise it's not really used. ;)
Actually the x system is implemented into Duolingo now, but still not a great habit to get into.